The present invention relates to a system for controlling the transmission ratio of an infinitely variable transmission for a vehicle, and more particularly to a system for increasing the transmission ratio at rapid acceleration.
An infinitely variable belt-drive transmission comprises an endless belt running over a primary pulley and a secondary pulley. Each pulley comprises a movable conical disc which is axially moved by a pressure oil servo device so as to vary the running diameter of the belt on the pulley depending on driving conditions. The system is provided with an oil pressure regulator valve and a transmission ratio control valve. Each valve comprises a spool which controls the pressure of the oil by shifting the spool.
The primary pulley has a Pitot pressure generating device for producing Pitot pressure dependent on engine speed. The Pitot pressure is applied to one axial end of the spool of each valve to bias the spool. On the other hand, the actual transmission ratio is detected by the axial position of the movable conical disc of the primary pulley which represents the running diameter of the belt on the primary pulley. The position of the movable conical disc is transmitted to the other axial end of the spool of the pressure regulator valve. Further, intake manifold pressure of the engine is applied to the other axial end of the spool of the transmission ratio control valve through a vacuum operated actuator so as to control the transmission ratio. Since the conventional control system comprises various mechanical devices, the construction is very complicated. In addition, the Pitot pressure increases like a curve of the second degree with an increase in the engine speed. Accordingly, it is difficult to exactly control the transmission ratio in a low engine speed range. Further, the intake manifold pressure also changes like a curve of the second degree with a change in the opening degree of the throttle valve of the engine. Therefore, the control operation at high engine speed can not be properly controlled. In addition, the characteristics of signals representing engine speed, manifold vacuum and pulley transmission ratio are roughly determined in accordance with engine operating conditions and cannot be corrected in dependency on various driving conditions of a vehicle.